To clone the rat CD5 gene we first produced two rat CD5 probes. The probes were obtained by polymerase chain reaction (PCR) on rat genomic DNA using primers designed on conserved regions between mouse and human CD5. The screening of a rat cDNA library at high stringency using these probes resulted in a 1.5-kb positive clone. The DNA sequence of this clone confirmed its CD5 nature, but the clone appeared to lack part of the 5' and part of the 3' end. These missing 5' and 3' ends were obtained by PCR on rat thymus RNA. By ligating these PCR products to the original 1.5-kb CDM8 clone, a full-length rat CD5 gene was constructed. The full-length clone showed high identity with mouse and human CD5; however, at the 5' site of the gene a region of 36 nucleotides is present which is not seen in either mouse or human CD5. We have evidence that this sequence is a normal constituent of the rat CD5 gene: first, it is in frame with the rest of the CD5 coding sequence; second, it does not contain a stop codon; and third, it is also present in the CD5 gene of other rat strains. We transfected the full-length CD5 construct in COS cells and demonstrated that indeed the CD5 protein is recognized by MRC OX19. Although we showed that CD5 mRNA is present in rat B cells, extensive flow cytometry analysis using MRC OX19 as antibody failed to detect B cells expressing significant levels of CD5 on their cell surface compared to other B cells in any tissue or cell suspension tested from a variety of rat strains. This is in contrast with the mouse where a distinct population of B cells (B-1a cells) can be found expressing more CD5 than the other B cells. Either B-1 cells are not present in rats or CD5 is not the right phenotypic marker for rat B-1 cells. It still remains to be investigated whether a population of B cells with functions similar to those of murine B-1 cells is present in rats.